Raise an error and print a more helpful message when file import fails

[djcsdy]

If swfmill attempts to import a font, but cannot find the file, it displays a warning similar to the following. Compilation does not fail.

WARNING: FreeType does not like GenR102.ttf

This is not very helpful. swfmill should fail and display a more helpful error message.

We should also improve swfmill’s behaviour when it fails to import other types of file.